The long-range objective of this research proposal is to determine what factors regulate the deiodination of thyroxine (T4) to 3,5,3 feet-triiodothyronine (T3) and 3,3 feet 5 feet-triiodothyronine (rT3) in different organs. Specifically, we wish to accomplish the following: 1) to define the mechanisms regulating the production of T3 from T4 in liver, kidney, muscle, and brain; 2) to determine what factors (hormones, metabolites, iodothyronines) are important in altering the production of T3 and rT3 from T4 in these organs; 3) to study the generation of alternate iodothyronine metabolites (rT3, di and monoiodothyronine metabolites, conjugates, etc) from T4; 4) to study the degradation of T3, rT3, and di and monoiodothyronines. To accomplish these goals, we will use in vitro perfusion techniques in liver, kidney, muscle and brain at near physiologic conditions to quantitate uptake of T4, production of T3, and the conversion of T4 to T3 and rT3. These measurements will be done using both radioimmunoassay methods and radioisotopic tracer techniques combined with high pressure liquid chromatographic (HPLC) separation of T4 and its iodothyronine metabolites. These techniques maintain the organ structure intact with the ability to respond to hormonal and substrate stimuli. Without these techniques, it would be difficult, if not impossible, to identify factors which alter the deiodination of T4 to T3 and rT3. These perfusion techniques will be used in three types of experiments. First, various hormones, metabolites, drugs, and iodothyronines will be added in vitro to perfusion medium, and their effects on T4 uptake, T3 and rT3 production, and the conversion of T4 to T3 and rT3 will be determined. Second, hormones, metabolites, drugs, and iodothyronines will be given to rats in vivo, and the effect on the above parameters will be measured. Third, abnormal metabolic states such as starvation, diabetes, hypothyroidism, and hyperthyroidism will be induced in rats, and the effect on T4 metabolism will be determined. These techniques will provide comprehensive information on the metabolism of T4 under physiologic conditions in both normal and abnormal states and will provide insight into those factors which regulate it.